The d24 today isn't so much more than a reminder of the somewhat older versions of Dungeons and Dragons, where they were used among other things for generating hours in a day. d24 dice were also used for special hit-to-rolls (hit confirmation rolls) when the attacker was somehow blessed or in advantage.
A d24 was basically a six sided die with 'bloated sides'. These sides jut out in four orthogonal triangles each, giving room to two odd and two even numbers. This makes room for 24 equally sized sides in which all opposing sides together equal 25.
Related Subjects: Armor Class, ThAC0, dice, Dungeons and Dragons, d6, d8, d12, d20, d100, RPG